Dispensers for absorbent sheet products are well-known in the art. Such apparatus comprise a store with an absorbent web which is to be dispensed. The web is conveyed with at least one conveying element for feeding the absorbent web to a position where it is cut so as to form separate absorbent sheet products for a user. In dispensers for absorbent material, like tissue material, a build-up of electrostatic charge can be observed. When two bodies of different material are in contact which each other, there is migration of electrons between the two surfaces. The number of electrons that migrate is dependent on the difference in the so-called work function of the two materials. The term “work function” stands for the energy required to remove an electron from the surface of a specific material to infinite. A material with a lower work function acts as a donor. From such donor material, the electrons migrate to the acceptor material with the higher work function. If the two bodies suddenly are separated from each other, the electrons try to return to their parent material. In the cases where the material is conductive, this is possible and the electrons migrate back to their parent material. However, if one or both of the two bodies are insulating materials, this will not happen. As a result, electrons get trapped in the surface of the material to which they have migrated.
Static electricity generates high voltages with low currents. Commonly accepted Standard IEC 61000-4-2 limits the allowable maximum voltage level to an amount smaller than +/−8000V. If the electrostatic charge exceeds such maximum voltage, it might affect other electrical components. Further, it is even possible that a user might be exposed to unpleasant discharges.
Various factors influence the build-up of electrostatic charges. The first factor is the type of material. In order to create an electrostatic build-up, two bodies have to be in contact with each other, where at least one of the bodies should be a bad conductor. When there are two bodies of dissimilar material it could cause the material to charge even more than when two similar materials are in contact with each other. This is the effect of the dielectric constant, or the work function. A material with high relative permittivity (the electric constant) becomes positively charged when it is separated from a material with low permittivity. A second factor is the contact area between dissimilar materials. The larger the contact area is, the more electrons migrate between the materials. As a result of this, a large contact area promotes a high electrostatic charge build-up. A third factor is the separation speed. The higher the speed of separation of the two materials is, the less is the possibility for the electrons to move back to the parent material. A higher separation speed results in a higher charge build-up. A further factor of influence is a possible motion between the materials. Firstly, the local heat generated by the friction between materials increases the energy level of the atoms making the escape of electrons easier. Secondly, a movement causes better surface contact by bringing the microscopic irregularities on both surfaces in contact with each other thus increasing the possibility of the electrons to migrate from one material to the other. The same applies for a higher temperature which results in easier release of electrons due to the higher energy level. Finally, atmospheric conditions can also influence the build-up of electrostatic charge. The more moisture there is in the atmosphere, the better is the ability of discharge. However, this is not true for all materials. For dispensers of the kind as stated above, however, the observation has been made that the electrostatic build-up tends to be higher in winter where the relative humidity of the ambient air is usually smaller.
Measurements show that the parts in a conventional dispenser which generate electrostatic charges are the conveying rolls and the knife or tear bar for severing the web into individual sheets. The paper leaves a dispenser positively charged so that the dispenser apparatus itself experiences a build-up of negative electrostatic charges.
U.S. Pat. Nos. 6,871,815 and 7,017,856 propose a system wherein a low impedance, high conductivity pathway, like a wire, is used to connect internal components of the dispenser that are subject to static charge build-up to a mechanical contact on the back of the dispenser housing. This contact, in turn, makes contact with the supporting wall upon which the dispenser is mounted, with the premise being that any static charge will be dissipated by the wall.
WO2008/053393 describes an electronic dispenser incorporating a passive, self-discharging static charge dissipating material incorporated with at least an internal component within the internal volume of the housing that stores static charge generated by operation of the dispenser. The web material is directed over the static charge dissipating material as it is conveyed through the dispenser in order to reduce the electrostatic load of the web material leaving the dispenser.